Rennet addition

FIGURE 17.16 Changes in rennet gels at various pH values (indicated) as a function of time after rennet addition, at 30°C. (a) Permeability B. (b) Endogenous syneresis pressure psyn. (After results by H. J. C. M. van den Bijgaart. Ph.D. thesis, Wageningen University, 1988.)... 

Creamer, L. K., Iyer, M., and Lelievre, J. (1987). Effect of various levels of rennet addition on characteristics of Cheddar cheese made from ultrafiltered milk. N. Z. J. Dairy Sci. Technol. 22, 205-214. 

Three types of coagulation were considered. The glucono(5)lactone (GDL)-induced coagulation system (GDL system) was prepared by acidification with 1.75 g/L GDL (Roquette, Lestrem, France). The rermet-induced coagulation system (rennet system) was prepared by the addition of calf rennet (SKW, Baupte, France) at 25.6/rg/L of milk. The GDL-I-rermet-induced coagulation system (mixed system) was prepared by acidification with 0.45 g/L GDL for 2h followed by the addition of rennet at 15.4/ig/L of milk. The experimental conditions for each system were selected in order to achieve gelation without syneresis. Coagulation kinetics were performed at 30°C. 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /Mactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 

The pH and the concentration of calcium in milk also vary, with consequential effects on the properties of renneted milk gels. The addition of CaCl2 to cheesemilk (0.02%) is widely practised and adjustment and standardization of milk pH by using the acidogen, gluconic acid-5-lactone (GDL), is recommended and commercially practised on a limited scale. 

Rennets. The traditional rennets used to coagulate milk for most cheese varieties are prepared from the stomachs of young calves, lambs or kids by extraction with NaCl (c. 15%) brines. The principal proteinase in such rennets is chymosin about 10% of the milk-clotting activity of calf rennet is due to pepsin. As the animal ages, the secretion of chymosin declines while that of pepsin increases in addition to pepsin, cattle appear to secrete a chymosin-like enzyme throughout life. 

Parmesan or Grana, as it is known in Italy, is a group of very hard bacteria-ripened, granular-textured cheeses made from partially skimmed cow s milk. They originated in Parma, near Emilia, Italy, hence the name. Special lipolytic enzymes derived from animals are used, in addition to rennet, to produce the characteristic rancid flavor. 

Procedures for extraction of chymosin from veils were described by Ernstrom and Wong (1974). Crude rennet extract contains active chymosin and an inactive precursor (prochymosin). Addition of acid to the extract facilitates conversion of prochymosin to chymosin and allows the extract to reach maximum activity. Even though activation at lower pH is faster, poor stability of chymosin below pH 5.0 in the pres-... 

Fungal proteases have been investigated extensively in search of suitable milk clotting enzymes. Patents have been issued for production of rennets from E. parasitica, M. Pusillus var. Lindt and M. miehei var. Cooney et Emerson. These have been approved in the United States as secondary direct food additives (FDA. 1984B) and have experienced considerable commercial success in the United States as milk-clotting enzymes for cheese manufacture. Many other fungal sources have also been tried in the effort to find an inexpensive replacement for chymosin. 

Proteolysis of casein begins with the addition of rennet to the milk and the formation of a coagulum. Calf rennet is actually 80% chymosin and 20% bovine pepsin A (Grappin et al 1985). Rennet can remain active in Cheddar and Camembert cheeses for up to three months, but... 

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